This invention relates in general to screen printing apparatus and more particularly to an improved thick film screen printing apparatus for the printing of conductive, resistive, and insulative films to the surfaces of ceramic or glass substrates.
Typically, the screen printing of ceramic substrates is accomplished by first applying a light sensitive emulsion to a mesh screen which is fastened by adhesive to a frame in a manner which places the mesh under tension. This provides that the screen fabric stays taunt within the frame. The light sensitive emulsion is exposed to a photographic positive of the pattern which is desired to be printed. The portion exposed to light undergoes a chemical change which makes it insoluble in water. The screen is then immersed in water and the unexposed portion of the emulsion is washed-out leaving openings in the mesh screen corresponding to the circuit pattern to be printed.
The screen with the printed pattern is mounted to a screen printing machine and the appropriate film in the form of an ink is applied to the screen. A squeegee is then drawn across the surface of the screen which forces ink through the emulsion pattern. The ink contacts the ceramic substrate surface printing the pattern found on the emulsion onto the substrate.
One particular type of printing used in the industry is termed off-contact printing. In the off-contact method the taunt screen is deflected approximately 0.040 inches during the printing stroke. Therefore, in the off-contact method, only the area of the screen contacting the squeegee at any one time is in contact with the substrate being printed.
One of the disadvantageous encountered with this particular method is that substrates are not entirely flat. The larger the substrate the greater the substrates warpage. In order to accomplish quality printing of ceramic substrates in the off-contact printing method a hard material is used for the squeegee. This material is generally neoprene or polyurethane of 70-80 durometer hardness. This hardness allows the squeegee to deflect the screen and force sufficient ink through the openings in the screen material. The pressure required to deflect the screen a break-away distance of 0.040 inch is approximately one pound per linear inch squeegee.
When screening large substrates the squeegee mentioned above is too hard to conform to the surfaces of warped substrates and consequently variations in the thickness and quality of the printing occurs across the face of the substrate. These variations can also be caused by the squeegee not conforming to previously printed patterns on the substrate.
Normally, the ends of the squeegee absorb the greatest force imparted by the deflected screen and therefore an unequal pressure is applied to the squeegee. After a number of uses the ends of the squeegee become rounded changing the shape of the squeegee and causes variations in the printed pattern. Particles of the squeegee which have been worn away enter the thick film ink contamination it, further causing printing variations.
A further disadvantage to the present technique of off-contact printing is the frictional effect of the squeegee on the screen and printing inks. As the squeegee travels across the screen the frictional force tends to pull the screen tauter behind it and loosens the screen in front of it. This effects the quality of the printing by dimensionally distorting the screen pattern. Additionally, the heat generated by the frictional force contributes to the vaporization of the ink thinner thereby changing the thixotropic nature of the ink. This causes additional variations in the printing quality.
Accordingly, it is the object the present invention to provide an improved thick film screen printing apparatus which can closely control the quality of printing on a ceramic substrates and overcome the above mentioned disadvantages.